Recent technical successes in isolating rat natural killer (NK) cells as a highly purified leukocyte subpopulation, in largely expanding the number of these cells in vitro by interleukin-2 (IL- 2) stimulation, and in demonstrating the release of natural killer cytotoxic factor (NKCF) by these cells, suggest that it is now possible to approach immunologically, biochemically, and molecularly the problem of NKCF characterization with good changes of success. The objectives of the proposed research are: a) to concentrate NKCF from culture supernatants of highly purified populations of IL-2-stimulated NK cells and to produce specific antibodies against it; b) to purify NKCF; c) to generate specific oligodeoxynucleotides deduced from the amino acid sequence of the isolated factor; d) to use these oligonucleotides as probes to test "selected" and "complete" cDNA libraries generated with the mRNA from the purified NK cells; e) to clone and sequence the NKCF encoding gene(s); f) to compare the sequence of NKCF to that of known cytotoxic proteins: g) to test the ability of recombinant NKCF to mediate killing of tumor cells in vitro.